Extremely Rare Finding of a Chiton (Mollusca, Polyplacophora) in a Rocky Intertidal Habitat in Nova Scotia (Canada)

Abstract

Chitons are a group of mollusks (polyplacophores) that differ from the mollusks that are most often found on marine rocky shores (bivalves and gastropods). On the Atlantic coast of North America, chitons have been reported in subtidal habitats from Newfoundland (Canada) to New England (USA). Here, we report the finding of Boreochiton ruber at the mid-to-high intertidal zone in a wave-exposed habitat in Nova Scotia (Canada). After surveying various coastal locations across Nova Scotia for 20 years (2004 to 2024), this was the only intertidal chiton we ever found in such habitats, making this an extremely rare occurrence. Rare species may contribute in unique ways to community functioning, making their reports valuable for biodiversity and ecosystem research.

Chitons are a group of mollusks (class Polyplacophora) that differ from the most common mollusks (classes Bivalvia and Gastropoda) found on marine rocky shores, so they add to biodiversity in a unique way. Many chitons are subtidal, although intertidal species exist as well. For example, on the Pacific coast of North America, intertidal chitons can be abundant and thus ecologically important [1,2]. On the Atlantic temperate coast of North America, however, chitons are thought to be subtidal. On this coast, chitons were found in subtidal habitats from Newfoundland, Canada [3,4], to New England, United States [5,6,7,8]. In Nova Scotia (Canada), chitons were reported for subtidal habitats from the Gulf of St. Lawrence [9,10,11,12], Atlantic coast [13,14], and Bay of Fundy [15,16]. Here, we report an extremely rare finding of a chiton at the mid-to-high intertidal zone in a wave-exposed habitat on the Atlantic coast of Nova Scotia.

As part of annual surveys of rocky-intertidal species along Nova Scotia’s Atlantic coast [17], we sampled West Point (43.6533 N, 65.1309 W) on 18 August 2017 at low tide (Figure 1). At the mid-to-high intertidal zone of a wave-exposed bedrock habitat, we found a chiton on a substrate that was covered mainly by coralline and non-coralline crustose red algae (Figure 2). The two most common chiton species reported for Nova Scotia are Boreochiton ruber (Linnaeus, 1767) and Tonicella marmorea (O. Fabricius, 1780) [9,10,11,12,13,14,15,16]. Boreochiton ruber was reported for cobble fields at 3–4 m of depth off McNutts Island and Cape Negro Island [13], near West Point. Both chiton species (which have been reported under various synonyms [18,19]) are morphologically similar but can be distinguished based on the size of the granules on their shells and dorsal girdle areas: B. ruber has small shell granules and large girdle granules, whereas T. marmorea has large shell granules and small girdle granules [20]. Our encountered specimen was examined and returned immediately to its habitat because of its rarity, so its shell granules were not obvious due to reflections on its then-wet shell (Figure 2 and Figure 3). However, the comparison of its girdle granules with published pictures for these two species [20] revealed that the size of the girdle granules of our specimen was similar to that reported for B. ruber and larger than for T. marmorea, indicating that our finding corresponded to a B. ruber specimen.

Organisms of this species are usually 1–1.5 cm long, occasionally reaching 2.1 cm [20]. Thus, the encountered organism was of adult size (about 2 cm in length; Figure 3). It may have been present at the intertidal zone after local recruitment from a pelagic larva, as crustose coralline algae are thought to enhance chiton larval settlement [14]. Alternatively, the chiton may have migrated to the intertidal zone in search of crustose red algae as food or to avoid subtidal predators such as American lobsters [21], which are common in the area [22]. Either way, this finding extends the known bathymetric habitat range for B. ruber on this coast. We have surveyed rocky intertidal habitats at various elevations, wave exposures, and locations in Nova Scotia for 20 years between 2004 and 2024 [17,23,24,25,26,27,28] and this was the only intertidal chiton that we ever found at a mid-to-high elevation in a wave-exposed habitat, making this rare finding remarkable. In 1170 intertidal plots surveyed along 350 km of coastline in 2008, we found 4 chitons, but they occurred in wave-sheltered habitats at low elevations. Rare species frequently remain unnoticed, but they are valuable to understand overall ecosystem function and resilience because of their uniqueness [29,30,31]. This notion emphasizes the value of reporting findings of rare species in natural ecosystems.